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<html lang="en">
<head>
  <meta charset="UTF-8">
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  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
   <input type="file">
   <img src="" alt="">
   <script>
      // 1. 获取dom对象
      const input = document.querySelector('input')
      const img = document.querySelector('img')

      // 2. 注册事件
      input.addEventListener('change', function(e){
         // console.log(e.target.files)
        // 2.1 生成本地url地址
        const url = URL.createObjectURL(e.target.files[0])
        
        // console.log(url)
        // 2.2 给img的src属性赋值
        img.src = url
      })

   </script>
</body>
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